A membrane electrode assembly can produce electrical energy by separating fuel oxidation, usually hydrogen, and oxygen reduction. The oxidation of hydrogen takes place at the anode in the presence of an anode catalyst. The electrons released in this process flow to the cathode via an external circuit while the protons formed migrate through the membrane and react with oxygen on the cathode catalyst to form water.
A membrane electrode assembly consists of a proton-conducting polymer electrolyte membrane, the two opposite faces of which are each coated with catalyst layers. To make electrical contact with the catalyst layers and to supply and remove the fuel, oxygen and water, gas distribution layers are laid onto the catalyst layers, these consisting of porous, electrically conductive, hydrophobized carbon substrates and a contact layer. The porosity of the carbon substrate is in the range between 50 and 95%. The average pore diameter is 30 to 50 μm and the thickness is between 100 and 400 μm.
The contact layers in the gas distribution layers improve contact between the catalyst layers and the porous carbon substrates. The contact layers normally consist of a mixture of a conductive carbon black and a hydrophobic polymer such as, for example, polytetrafluoroethylene (PTFE) and have a lower porosity than the gas distribution layers. They are often also called “microlayers” because their average pore diameter is less than 1 μm.
The proton-conducting materials in the polymer electrolyte membrane are also called ionomers. A tetrafluorethylene-fluorovinylether copolymer with acid functions, in particular sulfonic acid groups, is preferably used. Such a material is marketed by E. I. DuPont, for example, under the tradename Nafion®. However, other, in particular fluorine-free, ionomer materials such as sulfonated polyetherketones or arylketones or polybenzimidazoles can also be used. Polymer electrolyte membranes generally have a thickness between 30 and 100 μm.
The catalyst layers for the anode and cathode, on opposite faces of the polymer electrolyte membrane, contain a suitable catalyst which is dispersed in a porous layer consisting of an ion-conducting polymer and optionally a binder, wherein the ion-conducting polymer is usually the same as the polymer from which the membrane is also made. A fluorinated polymer, such as PTFE for example, is often used as the binder. The porous structure of the electrode layers ensures optimum three-phase contact between the ion-conducting ionomer, the catalyst and the gaseous reactants. This enables easy exchange of protons between the polymer electrolyte membrane and the active centers in the catalyst and leads to good electrochemical performance data for the fuel cell.
Noble metal blacks, that is finely divided particles of platinum or its alloys, or supported catalysts made of finely divided carbon particles, such as for example carbon black, on which the catalyst is deposited in high dispersion, are suitable as catalysts. The carbon substrates on the gas distribution layers mostly consist of a porous carbon fiber fabric, a carbon fiber non-woven or a carbon fiber paper. Carbon substrates are rendered water repellent by impregnating them with a dispersion of a hydrophobic material in order to avoid the condensation of water vapor in the pores of the substrate. A PTFE dispersion is often used for impregnating. After impregnation, the carbon substrates are heated to a temperature above the melting point of PTFE (about 340 to 390° C.). The purpose of the carbon substrates or gas distribution layers is to carry the gaseous reactants to the catalyst layers and to remove the water being formed at the cathode.
Many processes for producing membrane electrode assemblies of the type described above are known in the art. For example, a so-called Decal process is used to coat the polymer electrolyte unit with the catalyst layers. In this case, a protective film is first coated with a catalyst ink which contains a platinum supported catalyst and dissolved ionomer. The catalyst layer on the protective film is dried in an oven at 135° C. and then pressed at 145° C. under a pressure of between 70 and 90 bar onto a polymer electrolyte membrane. The protective film is then pulled off. Alternatively, the polymer electrolyte membrane can also be coated directly with the catalyst ink, in accordance with another process known in the art. In this case, coating takes place on the membrane heated to 160° C. and leads to a catalyst layer of high integrity and elasticity. Optionally, the polymer electrode membrane provided with the catalyst layers can be hot-compressed at 70 to 90 bar and a temperature of 185° C. To form a fuel cell, gas distribution layers which consist of a hydrophobized carbon substrate and a contact layer are laid on the catalyst layers.
Alternatively, it is also known in the art that the catalyst layers can be applied to the gas distribution layers, wherein these gas distribution layers are also provided with a contact layer of carbon black and PTFE prior to applying the gas distribution layers, in order to prevent the catalyst pastes from penetrating too deeply into the porous carbon substrate. Gas distribution electrodes are formed in this way and these are obtainable commercially, for example from the ETEK Co. To form a fuel cell, these gas distribution electrodes are applied to both faces of a polymer electrolyte membrane. As is known in the art, electrical connection of the electrodes to the membrane can be improved by impregnating the catalyst layers with, for example, a solution of an ionomer. The impregnated electrodes are dried before they are used to make up fuel cells.
Also known in the art is a gas distribution layer made of a carbon fiber fabric for membrane electrode assemblies. The carbon fiber fabric is also coated on the face turned towards the relevant catalyst layer, with a contact layer of carbon black and a fluoropolymer which is porous and water-repellent and is also electrically conductive and in addition has a fairly smooth surface. This contact layer preferably penetrates not more than half way into the carbon fiber fabric. The carbon fiber fabric can be pretreated with a mixture of carbon black and a fluoropolymer to improve its water-repellent properties.
Also known in the art is a gas distribution layer (here “intermediate layer”) which is obtainable by infiltrating and/or coating one face of a coarse-pored carbon substrate (carbon paper, graphite paper or carbon non-woven) with a composition of carbon black and a fluoropolymer, which reduces the porosity of the portion of the carbon substrate close to the surface and/or forms a discrete layer of reduced porosity on the surface of the substrate. The gas distribution layer is laid on membrane electrode assemblies with this coating against the catalyst layers. It is known in the art that the object of the coating, to form good electrical contact with the catalyst layers, is achieved in this way.
Coating the carbon substrates in accordance with methods known in the art with a contact layer of a carbon black/PTFE mixture is costly and requires subsequent drying and calcination at 330 to 400° C.
Based on the foregoing, there is a need in the art for a simplified and cost-effective process for producing membrane electrode assemblies that is suitable in particular for the processing of thin polymer electrolyte membranes. The resulting membrane electrode assemblies should form a compact, tightly bonded unit. There is also a need in the art for the membrane electrode assembly produced using the process.